The role of solvent quality and of competitive adsorption on the efficiency of superplasticizers in alkali-activated slag pastes

The loss of dispersing ability by polycarboxylates ether superplasticizers in alkali-activated slag cements has been widely reported. However, no clear-cut explanation of this phenomenon can be found to date. Therefore, the behaviour of poly(methacrylate-g-poly(ethylene glycol)) superplasticizers in NaOH or Na2CO3-activated slag pastes was investigated. The observed loss of efficiency of the polymer was not due to a specific property of the slag particles, nor to structural degradation of the polymer in the alkaline solutions. Actually, the ionic strength of the activating solution decreased the solvent quality and changed the polymer conformation, leading to a deterioration of the steric repulsion brought by the side-chains. Moreover, in the Na2CO3-activated systems, the adsorption behaviour of the polymers was also significantly altered. Here, this was not caused by a low calcium concentration or by a preferential adsorption of the superplasticizer on calcite crystallites. The most plausible explanation was a competitive adsorption with CO32− ions.